This invention relates to circuits for generating pulses across capacitive loads by means of a gated semiconductor switching device. More specifically, it is directed to a pulser circuit of the above noted type which is used to energize an electrostatic precipitator and which uses a thyristor or similar semiconductive switching element as part of the pulse generating circuit.
Electrostatic precipitators are electrical devices employed to remove particulate matter from a gaseous stream directed between oppositely charged precipitator electrodes. Precipitators are used in a number of industrial applications including chemical plants, and more particularly, including electric power plants and other potential sources of particulate solution. Recently electrostatic precipitators have been much more frequently employed than in the past because of the increased needs and desires for a particulate removal from gases vented to the atmosphere. It should also be appreciated that the electrostatic precipitators of primary concern herein are high energy devices typically consuming several tens of kilowatts of electrical energy. Accordingly, proper precipitator energization is important, not only with respect to particle collection, efficiency, but also with respect to economy and reliability of precipitator operation.
Many precipitator designs have been employed in the past. However, each basically operates upon fairly well established principles. Precipitators generally include a pair of conductive electrodes. Typically, one of the electrodes comprises parallel plain metallic sheets spaced a relatively small preselected distance apart which are operated at ground potential. Additionally, a planar array of wires, connected electrically together and disposed midway between and parallel to the conductive sheets, comprises the second electrode. This planar array is maintained at high electric potential. While it is possible to ground the wire electrodes and apply high potential voltage to the electrode sheets, this mode of operation is typically avoided for safety reasons. A number of the parallel plate electrodes are assembled in a housing which defines a plurality of parallel gas flow passages to the volume between the precipitator electrodes. These passages are also defined, at least in part, by the structure and arrangement of plate electrodes. In general, commercial precipitators employ a plurality of plate and wire grid electrode pairs. The area of a typical section of a precipitator may, in fact, possess up to about 30,000 square feet of plate electrode area. Naturally, such a configuration exhibits a certain amount of electrical capacitance between the wire and the plate electrodes. The capacitance of a typical precipitator section is on the order of between 0.05 to 0.15 microfarads. While the operation of such precipitators appears to be relatively simple, there are several phenomenon which occur, which can limit precipitator particle collection efficiency. Different methods of energizing precipitators significantly effect the amount of electrical energy and power expended in removing a given fraction of particles from a gas stream in a given precipitator and with a given type of particle. It should be pointed out that precipitators generally operate at peak voltages of between 40,000 and 80,000 volts and each section may draw a current of about 1.5 amperes. It is thus easily seen that precipitator power levels of 80 kilowatts are not uncommon.
Therefore, electrical efficiency is a significant economic factor in plants employing electrostatic precipitators for the removal of particulate matter. Furthermore, for continuous plant operation, reliability of the precipitator and precipitator energizing components is very important. In normal operation, particulate matter in the gas to be treated acquires a negative charge as the result of induced ionization effects occurring principally in the vicinity of the precipitator cathode wires. The charged dust particles are then attracted to the precipitator anode plates where a layer of anode dust accumulates. As this dust layer accumulates, an increasingly thick dust layer is formed on the plate (that is, sheet) electrode. Even though the dust layer may be periodically removed by means of vibrating, wrapping, or otherwise flexing the anode plates, there is still an efficiency reduction concomitant with the formation of this highly resistive layer. Accordingly, efficient yet effective economical ways of energizing precipitators are highly desirable, particularly for the collection of dust particles exhibiting high resistivity, that is a resistivity of about 10.sup.-11 ohm-cm, or higher. For example, such dusts are created in the burning of low sulphur coal used by the electric utility industry.
The design and operation of electrical circuits for precipitator energization has taken many different paths in hope of arriving at a method of precipitator operation which is efficient, reliable, controllable and relatively inexpensive to implement. Furthermore, there has not been broad agreement amongst the practitioners in this art with respect to optimal precipitator energization methods. Of late, various pulse energization methods have come into broad use. An inherent problem in using pulse energy to drive a large capacitive load, as is typically presented by large capacitive precipitators, is that a considerable amount of energy is required to repetitively charge the load capacitance to a high voltage level. Since the capacitive load dissipates relatively little energy as a result of each applied pulse, recovery of the remaining energy stored in the load, and not consumed by for example, corona discharge or arcing, is of prime economic importance. Inverter circuits for accomplishing the above typically use a thyristor shunted by a feedback diode, the diode being connected in inverse parallel relationship across the thyristor. The diode is needed to carry reactive energy to the supply during some portion of the operating period. The current flowing through the capacitive load as a result of using such an arrangement takes on a generally sinusoidal waveshape. A cycle is initiated by triggering of the thyristor which conducts for the first half of the sinusoid, current through the thyristor rising to a peak and then falling to zero at the time voltage in the load reaches its peak value. During the second half of the sinusoid, current through the thyristor is cut off and the device is substantially in a nonconductive state while the diode shunts current around the thyristor in a direction opposite that which it flowed during the first half of the sinusoid. After diode conduction ceases, the thyristor must be capable of blocking a high applied forward voltage until the arrival of the next trigger pulse on its control electrode to begin a new cycle of operation.
During the operation of such capacitive loads, such as electrostatic precipitators, sparkovers can occur from time to time. A sparkover is any momentary low impedance condition in the secondary of the load transformer such as a sudden electrical arc or an electrical short created across the load terminals. Such sparkovers can occur during both the rise of the applied voltage pulse across the precipitator and during the decay of that pulse. If a sparkover occurs during the time when the current is flowing through the diode (the decay of the voltage pulse across the load), a rapid decrease in current leading rapidly to current reversal through the diode occurs, followed by a rapid rise in forward voltage across the thyristor. If such change in foward voltage across the thyristor occurs during its forward recovery or turn-off time, hereinafter designated as t.sub.q, i.e., when the thyristor is not capable of blocking such forward voltage, the thyristor will be forced to conduct under circumstances which may damage or destroy it. Specifically, if a thyristor is forced into conduction during the time when there is insufficient gate current to provide a sufficiently large conducting channel through the device, the result is a weak turn-on, i.e., a large current flow through a relatively narrow conducting channel resulting in overheating and potential destruction of the device.
One method for protecting a thyristor switch in an electrostatic precipitator under such circumstances is described in U.S. Pat. No. 4,503,477 wherein it is suggested that a thyristor switch element within a pulse generator supplying a capacitive load may be protected against damage during sparkover which occurs in the load by triggering the thyristor immediately after the sparkover occurs. According to this prior art teaching, the thyristor is triggered into conduction if the slope of the voltage across the load exceeds a preset positive value during a period of pulse decay. Also, this patent describes another alternative for detecting sparkover by sensing the pulse current, which consists of a negative half-period and a positive half-period, and triggering the thyristor to conduction when a shift to negative occurs during the normally positive half-period. Significantly, for both methods disclosed in the above noted patent, it is noted that firing of the semiconductor device into a conductive state after sparkover should take place within 15 microseconds, preferably in less than 2 microseconds.
It has been found, however, that failure can still occur during particular times in the pulser cycle even if firing the SCR to conduction is accomplished within a period of less than 1 microsecond, and this is mainly due to the uncertainty of the device characteristics during the above-noted t.sub.q time interval. The need for a short time interval for switching the thyristor into a conductive state is especially critical at the time of transition from thyristor conduction to diode conduction. This transition coincides with the peak voltage across the load and the beginning of the decay from this peak.
In the usual operation of a pulsed electrostatic precipitator incorporating a technique for protecting the thyristor similar to that suggested in U.S. Pat. No. 4,503,477, the thyristor is, nevertheless, particularly vulnerable to such potential damage at several times during the operating cycle. Firstly, it is particularly vulnerable to sparkover damage near the peak of the voltage applied across the precipitator electrode (near the transition between thyristor conduction and shunting diode conduction) since at this time the value of applied forward voltage impressed across the thyristor switch as a result of a sparkover is at a maximum. In addition, since current has not yet begun to flow in great magnitude through the shunting diode, and the thyristor has not yet regained its forward blocking capability, current through the thyristor increases rapidly in response to this applied positive forward voltage. It is difficult to react to a sparkover in a sufficiently short time to protect the thyristor from weak turn-on under such conditions. The vulnerability of the switching element of a pulser circuit during the time period occurring near the peak precipitator voltage is specifically addressed in concurrently filed, commonly assigned, patent application Ser. No. 761,459, entitled "Protection Arrangement for Switching Device of a Capacitive Load Pulser Circuit", in which there is disclosed a reliable technique for firing of the SCR to protect it from damage as a result of sparkover occurring near the above noted transition by insuring that a strong firing gate pulse is always present during the time the thyristor is within this transition period, i.e., near peak load voltage or the transition from thyristor conduction to diode conduction, independent of the detection of a sparkover.
Another point of vulnerability in such pulser systems is near the end of a pulser cycle when forward voltage rises sharply across the thyristor during normal operation, i.e., operation without sparkover. If the thyristor is still in its t.sub.q or turn-off time period at this time, the ensuing weak turn-on will damage the thyristor. In systems of the type disclosed in aforementioned U.S. Pat. No. 4,503,477, there is a possibility that a trigger pulse will be falsely sent to the thyristor gate so near to the end of the cycle (when forward voltage returns) that charge in the thyristor gate has sufficient time to diminish to a level such that a weak turn-on is initiated by the return of forward voltage. False triggers may be the result of electrical noise or signals coupled from adjacent circuits. This problem due to false triggering is the subject of the instant application.